Delivery devices for administering a beneficial agent to a biological fluid environment of use are known in the prior art. Representative examples of various types of delivery devices are disclosed in U.S. Pat. Nos. 3,845,770; 3,916,899; 3,995,632; 4,111,202; 4,111,203; 4,203,439; 4,327,725; and 4,612,008, which are incorporated herein by reference. The delivery devices described in the above patents operate successfully for their intended use and they can delivery many beneficial agents for their intended effects. However, it has been observed that their use can be limited because they lack the necessary elements to delivery beneficial agents that are sensitive to fluids and to fluids containing biological gases. Their use may be limited because beneficial agents that are sensitive to such aqueous biological fluids or to other fluids external to the delivery device may be adversely affected by fluids that enter the device and contact the beneficial agents during operation of the device. Examples of such fluid-sensitive agents include proteins, peptides, and hormones. Moreover, the prior art devices lack the necessary means for their use as implant devices for dispensing such sensitive agents to a biological fluid-rich environment of use.
To overcome the limitations associated with the prior art delivery devices, a delivery device has been developed and is described and claimed in copending, commonly-assigned U.S. Pat. No. 5,034,229, filed Dec. 13, 1988, to Magruder et al. for Delivery System Comprising Means for Governing Fluid Ingress into the System, the entire disclosure of which is incorporated herein by reference. This delivery device comprises a compartment, one portion of which is impermeable to fluid and contains a fluid-sensitive drug protected from a fluid environment and a second portion of which is permeable to fluid and contains an expandable driving member for administering the drug to the fluid environment of use. The system has been found to be particularly useful as an implant in livestock for delivering a fluid-sensitive drug over a broad range of dosage delivery rates according to a predetermined time-release pattern.
Although in vitro tests and in vivo tests on isolated animals indicated satisfactory system performance, in vivo tests under field conditions of the delivery device of U.S. Pat. No. 5,034,229 in livestock demonstrated an undesirably high failure rate, either by failing to deliver the beneficial drug at the desired rates or by failing to deliver the required dosage of the drug or by the fluid-sensitive drug coming into contact with fluid prematurely and becoming adversely affected prior to its delivery into the fluid environment of the host animal. The discrepancy between in vivo tests on isolated animals and on animals under field conditions was totally unexpected, not readily explained, and could adversely affect the commercialization of the delivery device.